The concept of heating a fluid stream and sensing its temperature to thereby obtain a measure of fluid flow it well known. For example, in U.S. Pat. No. 946,886 issued on Jan. 18, 1910 to Carl C. Thomas, there is disclosed an arrangement wherein a heater is inserted into a flow stream, and temperature sensors are located upstream and downstream of the heater. In one mode of operation Thomas measures the heater current necessary to maintain a constant temperature different between the two sensors. However, since Thomas relies on the movement of fluid between the two sensing points to establish the temperature differential, his arrangement is inapplicable to situations wherein the flow approaches zero.
A significant advance in thermal flowmeters is disclosed in U.S. Pat. No. 3,181,357 which issued to James M. Benson on May 4, 1965 wherein the actual flow conduit serves as both the heat source and a portion of the temperature sensing arrangement. Benson inductively couples a constant amount of power into the conduit and uses the conduit as one element of a thermocouple to measure changes in fluid temperature as heat transfers between the heated conduit and the fluid stream. The Benson flowmeter is inherently non-linear because the measured variable is differential temperature which is inversely proportional to flow. This significantly limits its usefulness and its range of measurement. Furthermore, the Benson device requires a relatively high operating temperature in the conduit for adequate flow sensitivity (typically 213.degree. C. above ambient for a 10 mvdc thermocouple signal) which prevents its use with some fluids, notably liquids with boiling points below 240.degree. C. and gases whose physical properties change adversely at elevated temperatures.
Also known are thermal devices, commonly referred to as "hot wire anemometers", which have been made to operate in the constant temperature mode and used to measure fluid flow by placement inside a conduit. Such devices measure the cooling effect of a fluid on a heated body (wire, thermocouple, etc.,) submerged in the fluid stream; that is the velocity of the fluid at the point of insertion. The calibration of the device is strongly dependent on the thermal conductivity of the fluid and the relationship between power and flow is not linear. Furthermore, this type of thermal device is subject to damage from streaming debris in the conduit, and in the case where the fluid is combustible, an ignition problem is presented.